Facsimile (fax) machines are used to transmit images of printed documents from one place to another. Typical fax machines are capable of both sending and receiving transmissions. Fax machines use bi-directional digital communication signals, represented as audio tones transmitted across standard telephone lines. Fax transmission techniques are in the same general family as modem (modulator/demodulator) data communication used by computers and data processing systems, and it is possible for the modem of a computer system to support data communication between the computer and a fax machine, in addition to computer-to-computer data communication. The standards that govern interoperation of fax systems are set and governed by the CCITT.
Typically, a fax transmission is point-to-point, as is illustrated in FIG. 1. One fax machine (the transmitter) places a standard telephone call to the telephone line assigned to a second fax machine (the receiver). If the receiver's telephone line is shared with a voice phone or other functions, then one of a variety of methods is used to answer the call and determine that it is a fax transmission and should be directed to the receiver fax machine, as opposed to a voice or other data call. Once the call is answered by the receiver, the receiver and transmitter enter into a “negotiation” to determine the maximum data transmission rate supported by the transmitter, the receiver, and the intervening telephone line, as well as the usage of other features and protocols that the systems may optionally support. After the connection is completely established, the transmitter sends the data representing the contents of each page of the fax transmission to the receiver, where the data may be temporarily buffered and is printed. The transmitter and receiver have an ongoing handshake protocol to prevent the data buffers of the receiver from overflowing, which may happen if the receiver cannot print the pages received as quickly as the data defining them arrives.
As illustrated in FIG. 2, a conventional fax device 200 includes a controller/sequencer 201, a scanner subsystem 202, a paper intake transport mechanism 203, a printer subsystem 204, a blank paper transport mechanism 205, front panel controls 206, and a telephone communication subsystem 207. The controller/sequencer 201 is typically an embedded microprocessor system, and implements the communication protocols and generally controls the fax system device 200. It also contains a memory buffer subsystem 208 in which data is stored prior to transmit or after receipt. The scanner subsystem 202 is typically an optical scanner that “reads” the surface of the page(s) to be transmitted and creates an equivalent digital bitmap image. The paper intake transport mechanism 203 is a mechanical system for taking in the page(s) to be transmitted, passing them through the scanner subsystem, and placing them in an outfeed tray. The printer subsystem 204 is a printer, such as a thermal or ink jet printer or other digital print device that creates hard copy documents based on the page image data received. The blank paper transport mechanism 205 is a mechanical system for taking blank paper from a reservoir, passing them through the printer subsystem, and placing them in an outfeed tray. The front panel controls 206 is a user interface to the fax system, typically including an array of buttons or keys and a display such as a liquid-crystal or electroluminescent display. The telephone communication subsystem (telephone line interface 207) serves as an interface between the fax machine to the telephone line and other fax machines. The constituent components of the conventional fax device 200, such as the scanner, printer, and controller, are also capable of being configured as a copy machine. Documents to be copied may be scanned as if they are to be faxed, and the scanner data is instead printed out to make a copy of the original document.
In addition to the basic fax machine shown in FIG. 2, many existing systems integrate fax capability with other functions. For example, there are so-called “multi-function” or “all-in-one” systems that take advantage of the fact that a fax machine contains printer and scanner subsystems. A multi-function fax has an interface to a computer, and serves as a printer and/or scanner for the computer as well as operating as a stand-alone device or a fax machine remotely controlled from the computer. Other fax machines integrate the functions of a telephone and/or a telephone answering machine, sharing their telephone line connection between these functions and the fax function. This can be advantageous as it is difficult for an answering machine and a fax machine to share a telephone line if they are not integrated. Finally, a large number of modems for computers, virtually all modern “internal” modems, also have the ability to support the fax data modulation schemes and communication protocols. Typically, the software for these modems allows fax “pages” to be transmitted directly from the software applications of the computer in lieu of printing actual paper pages and then feeding them through a separate fax machine, and can receive fax data which is then displayed to the screen and/or printed on the printer(s) connected to the computer. In all of these cases, the quality that fundamentally makes a device a “fax machine” is its ability to communicate using the standard fax protocols, rather than particular physical components like the paper transport mechanisms, internal scanner or printer.